This invention relates to lens supports. More particularly, this invention relates to lens supports having a lens sleeve that is designed to float in a lens holder and that is capable of resting on a film holder, the lens support also having a lens capable of sliding axially in the lens sleeve through provision of a focussing device operated by a servomotor. Lens supports of this kind are commonly used in, for instance, microfilm readers and are hence generally known.
Due to the lens sleeve being capable of sliding on the film holder, the distance between the microfilm and the lens remains constant even in the case of relative movement of film holder and lens, provided that the thickness of the upper, transparent plate of the film holder is constant and that the microfilm lies level in the film holder. Practical operation, however, necessitates refocussing when changing films with different emulsion positions. This is particularly true of automatic film change.
With the lens supports known to date, the focussing device driven by the servomotor is fixed at the lens holder or at the unit housing. Since the lens sleeve moves in relation to the lens holder, provision has to be made of movable transmission elements positioned between the lens sleeve and the focussing device, thereby maintaining the capability of providing such relative movement.
These movable transmission elements involve a high degree of design expenditure, bring about undesired play in the focussing device and may be the cause of lens jamming.